Steverustite, Pb₅²⁺(OH)₅[Cu⁺(S⁶⁺O₃S^2–)₃](H₂O)₂, a new thiosulphate mineral from the Frongoch Mine Dump, Devils Bridge, Ceredigion, Wales: description and crystal structure

Abstract Steverustite, ideally Pb 5 2+ (OH) 5 [Cu + (S 6+ O 3 S 2– ) 3 ](H 2 O) 2 , is a new supergene mineral from the Frongoch mine dump, Devils Bridge, Ceredigion, Wales. It generally forms fibrous fan-like bundles that occur in small cavities in quartz veins with other supergene species. Crystals are fibrous to acicular, elongated along [010], and are bounded by ( h 0 l ) faces too small to index reliably. It is transparent, colourless to white with a white streak, has a vitreous lustre, does not fluoresce under ultraviolet light and is brittle with a splintery fracture. The calculated density is 5.150 g/cm 3 , and the calculated mean refractive index is 1.94. The mineral is monoclinic, P2 1 /n, a 12.5631(7), b 8.8963(5), c 18.0132(11) Å, β 96.459(1)º, V 2000.5(3) Å 3 , Z = 4, a:b:c = 1.41217:1:2.02480. The seven strongest lines in the X-ray powder diffraction pattern are as follows: d (Å), I , ( h k l ): 3.934, 10, (Ī14); 3.934, 8, (Ī11); 3.348, 7, (313); 6.211, 6, (200); 4.797, 6, (211); 3.026, 6, (314); 2.837, 5, (016). Chemical analysis by electron microprobe gave PbO 72.59, SO 3 15.78, Cu 2 O 4.47, S 2– 6.32, H 2 O 4.83, O=S 2– –3.15, total 100.84 wt.% where the amount of H 2 O was determined by crystal-structure analysis. The resulting empirical formula is Pb 4.99 2+ Cu 0.96 + (S 6+ O 3 S 2– ) 3.03 (OH) 4.88 (H 2 O) 1.67 , based on O + OH + H 2 O + S 2 – = 18.67 a.p.f.u. (atoms per formula unit) with H 2 O = 1.67 a.p.f.u. (from crystal-structure solution and refinement). The crystal structure of steverustite was solved by direct methods and refined to R 1 = 2.7% for 3366 unique (Fo > 4σF) reflections. There are five distinct Pb2+ cations with coordination numbers from [8] to [11], all of which show stereoactive lone-pair behaviour and which form a strongly bonded cluster of composition [Pb 5 (OH) 5 ]. There is one Cu + cation triangularly coordinated by three S2– atoms that belong to three thiosulphate groups, forming a Cu + (S 6+ O 3 S 2– ) 3 group. The [Pb 5 (OH)5] units and [Cu(S 2 O 3 )3] groups occur at the vertices of interpenetrating 3 6 nets to form layers of composition [Pb 5 (OH) 5 Cu(S 2 O 3 ) 3 ] parallel to (010) which are linked by weaker bonds. Examination of the stereochemistry of thiosulphate and thionate structures shows that the combination of <S–O> and S–S distances are distinct for these two types of structures.

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